Hundreds of protestors carried an effigy of special advisor to the United Nations Representative for Children and Armed Conflict, Allan Rock, and shouted slogans against his allegations made last week.
Marxist JVP legislator Wimal Weerawansa said Rock's charges against Sri Lankan security forces were unprecedented and formed part of a "conspiracy" to discredit the government.
"It is the first time in history that security forces have been accused of involvement in recruiting child soldiers," Weerawansa told reporters outside the UN offices here.
"This is a conspiracy and we are here to show that we will defeat it.
"
Sri Lanka's Tamil Tiger rebels have long been accused of recruiting children as combatants, but Rock's remarks were the first official charge against the island's military of doing the same.
"Is whitewashing the Tiger terrorists the obligation of the UN?
, asked one placard carried by a Buddhist monk.
"Is Allen Rock a Tiger," asked another.
Sri Lanka's state-run Dai